This paper describes the effects of pressure and postmortem aging treatments on the in situ postmortem activity of rabbit and bovine muscle. Rabbit proteasome retained their ability to hydrolyze synthetic peptides in muscle exposed to pressures of up to 100 MPa. However, when pressures of 400 MPa or more were applied, proteasome were markedly inactivated. The ability of the bovine proteasome to hydrolyze synthetic peptides was retained in muscle exposed to pressures of up to 200 MPa. However, when a pressure of 500 MPa or more was applied, the proteasome were markedly inactivated. We found that proteasome were difficult to extract from excessively pressurized meat. The results of this experiment show that muscle that is exposed to low-pressure pressurization retains its ability to hydrolyze synthetic peptides. This finding implies that proteasome are involved in the high-pressure tenderization process, in combination with other intramuscular proteinases.